Gas detection devices are used, for example, in plants for delivering and processing combustible and/or toxic gases in order to detect unintentionally discharged gases.
Gas detection devices are known that measure the concentration of such gases locally, i.e., in the immediate surrounding area, and are interlinked with one another in order to make it possible to monitor larger areas.
In addition, there also are gas detection devices with an open measured path, which are called open-path gas detection devices. The measured paths may range from a few meters to a few hundred meters. Open-path gas detection devices analyze electromagnetic or light radiation, which has passed through a defined monitoring area. The electromagnetic or light radiation is analyzed with respect to a possible interaction with an unintentionally discharged gas, which interaction causes changes in the properties of the radiation. This makes possible a continuous monitoring of a relatively large area in respect to an unintentional discharge of gases, and the quantity and the species of the discharged gas or gases can also be inferred by analyzing the changed properties of the radiation. The radiation used for the monitoring may be, for example, thermal radiation of the background, or this originates from a radiation source of the gas detection device itself.
The functional devices used in gas detection devices have, as a rule, a limited field of view, i.e., these must be aligned relatively accurately towards the radiation source or the area to be monitored. This also applies to open-path gas detection devices, which comprise as functional devices, as a rule, at least one transmitter with a radiation source and a receiver with a radiation detector, to which the radiation emitted by the radiation source is focused. The radiation source may be, for example, a thermal radiator, for example, a xenon flash lamp, or a semiconductor radiator, for example, a tunable laser. Such open-path gas detection devices require that both the transmitter and the radiation source integrated therein be aligned in order to direct sufficient radiation output to the entry aperture of the receiver and also that the receiver be aligned in order for the radiation falling on the entry aperture to reach the radiation detector as centrally as possible.
Another type of open-path gas detection devices comprises a reflector, which is positioned at a distance of usually up to 50 m from a combined transmitter/receiver unit and to which the radiation emitted by a radiation source of the transmitter is projected. The reflector reflects the radiation in the direction of the transmitter/receiver unit, as a result of which this can be detected by a radiation detector of the unit.
What is detected in open-path gas detection devices is, in principle, not the concentration of the detected gas but the so-called integral concentration. This is the product of the concentration and the length of the measured path. Localization of a corresponding gas leak, i.e., determination of the concentration and size of the gas cloud, is not possible, because a large cloud with a low concentration may lead to the same measured value as a small cloud with a high concentration. The fault indications of open-path gas detection devices are therefore rarely used alone to trigger follow-up measures, for example, an emergency shut-off or evacuation of the plant being monitored. As rule, at least an additional confirmation of the detection by spot measuring devices, which can measure the absolute concentration, is needed. The measured area causing the fault also often must be entered by emergency crews, e.g., firefighters, in order to establish the cause of the fault indication.
Open measured paths may, moreover, be blocked by obstacles, e.g., vehicles or persons, or blocked by contamination of the functional devices of the gas detection device and the detection of the radiation may thus be interrupted (so-called “beam block”). As a rule, a fault indication is issued only in case of a rather long-term interruption of the measured path, because it can no longer be assumed in this case that only a person or a vehicle is moving through the measured path.
The cause should be able to be established as quickly as possible when a “beam block” or a concentration of a gas to be detected above a limit value is determined.
A fire alarm box for monitoring a room and for triggering a fire alarm is known from DE 10 2010 015 467 B4. This box comprises a smoke sensor and a temperature sensor, and the measurement results of the smoke sensor and the temperature sensor are linked in order to avoid false triggering as much as possible. The fire alarm box may have, in addition, a camera, by which the room can be seen optically for monitoring for break-in. A picture taken by the camera can be transmitted for this via a radio network. Furthermore, it is disclosed that the fire alarm box may have a gas sensor, which is used to detect carbon dioxide formed during a fire.
DE 20 2008 010 173 U1 discloses a mobile monitoring center for indoor and outdoor use. The monitoring center comprises a steel cylinder with an attached mast, which has at least one sensor system and an auxiliary device, the steel cylinder comprising a supply unit with a power supply from the power line or from a battery. Furthermore, a control unit and a device for online communication shall be present. The sensors shall be image-recording devices, a motion sensor and a stereometric sound-recording device. The possible use of a module for gas detection is also disclosed. The auxiliary devices shall comprise a searchlight and devices for optical and acoustic signal generation.